Shot peening is a metal-finishing operation in which small steel shot is directed against the workpiece being treated. The impact of the shot on the workpiece plastically deforms the surface thereof to a depth of a few thousandths of an inch, producing residual compressive stress. The workpiece is thus made more resistant to fatigue failure. Surface hardness of the workpiece is also increased by the cold working produced by the shot. The shot is hurled against the workpiece at high velocity by centrifugal action or by an air blast.
The two most common failures of metal are due to plastic deformation and fracture. Both these failures are caused by tensile strain. Plastic deformation is caused by the weakening of the metal under a steady load, and fracture is caused by an actual break in the crystalline structure of the metal by repeated flexing. The prior art shot peening process results in a compacting of the surface of the metal which enhances its resistance to both of the aforesaid causes of failure.